The present invention is directed to a method and apparatus for removing contaminants from gases. Specifically, the present invention is directed to a process for removing mercury and entrained particulate matter from gases prior to liquefaction and a novel guardbed for practicing that process. The present invention is particularly useful as a means for removing particulate matter and residual amounts of mercury from a natural gas stream prior to liquefaction.
Natural gas produced from natural gas wells is usually separated and purified over several steps to provide products for a variety of end uses, see FIG. 1. The natural gas well stream exiting the well is a high-pressure mixture and is typically sent to a separator vessel or series of separator vessels maintained at progressively lower pressures where it is separated into a gaseous fraction and a liquid fraction.
The gaseous fraction leaving the separator is then directed to a gas treatment and purification plant where impurities such as hydrogen sulfide, carbon dioxide and mercury are significantly reduced from the stream. At this stage, the mercury concentration is normally reduced to levels of less than 0.1 micrograms/Nm.sup.3, the hydrogen sulfide concentration is reduced to about one (1) part per million and the carbon dioxide level is reduced to the parts per million (ppm) level.
The gaseous fraction may be purified by passing it over a bed of activated carbon which has been impregnated with sulfur. The gas reacts with the sulfur and the mercury content of the gas can be reduced from about 250 .mu.g/Nm.sup.3 or higher to levels approaching 0.1 .mu.g/Nm.sup.3.
The gas may be further purified by treating the gas with a hot aqueous potassium carbonate solution to reduce the carbon dioxide and hydrogen sulfide content of the stream. For example, the carbon dioxide of the gas can be reduced from about 15% to about 0.3% and the hydrogen sulfide content of the gas from about 80 ppm to about 6 ppm.
Still further purification of the natural gas stream treated with the carbonate solution may be desirable. Further treatment to reduce the amount of carbon dioxide and hydrogen sulfide in the gas may be achieved by treating the gas with an amine solution such as an aqueous solution of diethanolamine. Diethanolamine has the ability to absorb carbon dioxide and hydrogen sulfide and can reduce the carbon dioxide content of the gas from about 0.3% to about 50 ppm and the hydrogen sulfide content from about 6 ppm to about 1 ppm. The natural gas is then washed with water to remove traces of the entrained absorptive amine solution.
The washed natural gas is water-saturated and has to be dried prior to liquefaction. Usually, drying is achieved by contacting the wet gas with a desiccant in a packed bed specifically designed for this purpose. The desiccants found o in the drying beds used in LNG plants may include any solids which have the ability to absorb water and release it upon heating to regenerate the desiccant and have the ability to withstand the regeneration temperatures of from about 400.degree. to about 700.degree. F. Examples of the desiccants include alumina, silica, silica-alumina, molecular sieves, silica gels and combinations thereof. The desiccant bed undergoes repeated cycles of absorption and regeneration. To ensure that the desiccant bed retains its integrity during the drying and regeneration cycles, a protective layer containing, for example, inert alumina spheres having a depth of about 0.5 to about 2 feet may be placed over the desiccant.
The high pressure of the gas stream contacting the desiccant bed and the repeated cycles of adsorption and regeneration eventually take a toll on the drying bed. Fragments of the desiccant materials and/or protective layer materials can break away and be carried downstream, ultimately having a negative effect on the liquefaction process.
Separately, in spite of the above-described purification steps, residual amounts of mercury may remain in the gas stream in amounts of about 0.1 .mu.g/Nm.sup.3. The processing of natural gas in LNG plants requires that the gas contact processing equipment made primarily of aluminum. This is particularly true during the liquefaction steps where the gas is cooled in aluminum heat exchangers. Under certain process conditions, the residual amounts of mercury present in the gas stream can amalgamate with aluminum. This amalgamation is cumulative and can cause severe corrosion which results in operational problems. Further, since aluminum heat exchangers represent a capital investment of several million dollars, damage to these exchangers is to be avoided if at all possible. Damage such as corrosion cracking of the heat exchangers can lead to equipment failure. Repair to the system is often difficult because the amalgamation may effect the welded seams of the aluminum materials.
Guardbeds which are designed to protect downstream liquefaction apparatus by removing the residual Hg from the gas stream, trap entrained particulate matter. These guardbeds, however, can become clogged with the entrapment of the particulate matter. Such clogging usually causes a pressure drop in the system and tends to reduce production efficiency.
In view of the foregoing, improving natural gas processing methods and apparatus is desirable. In particular, it would be beneficial to provide a mechanism for protecting the liquefaction equipment by reducing the levels of residual mercury from the gas prior to it entering the liquefaction equipment while at the same time providing a mercury removal bed which has a long, stable life cycle. This is achieved by providing both a mercury removal function and improved resistance to pressure drop build up within the bed caused by entrained particulate matter.
It is, therefore, an object of the present invention to provide an improved method and apparatus for removing contaminants such as residual amounts of mercury and entrained particulate matter from a gas stream in order to preserve the integrity of liquefaction apparatus such as aluminum heat exchangers and maintain high throughput.
It is a further object of the present invention to remove the above-described contaminants from the gas without incurring an unacceptable pressure loss in the system.